Fall-applied nitrogen (N) is always a topic of interest among cool-season turfgrass managers. It’s a fairly common practice, yet not a lot of research examines its benefits (or detriments).
Given that, researchers at the University of Minnesota and the University of Wisconsin decided to take a look at late-season N fertilization, conducting a literature review to examine what the research had to say about the efficiency and use of fall-applied N in cool-season programs.
One of the first items they noted was that N has six basic routes in our plant/soil system: (1) It can be harvested in clippings; (2) it can be stored in soil, thatch, organisms or microbial communities; (3) nitrate can leach downward; (4) nitrate can run off from the surface; (5) it can denitrify and be lost to the air as gaseous N; and (6) ammonium can volatilize as ammonia gas.
The last four items in that list are of environmental concern, and substantial turfgrass research has shown that if N sources, rates and timings are managed properly, the environmental fate of applied N to water and air can be greatly reduced.
But what has research shown for fall N recommendations and how fall N can affect color and quality of cool-season turfgrasses?
Well, it is generally recommended that fall N be applied shortly after turfgrass shoot growth ceases. This allows the grass to partition photosynthate and assimilated N into roots, stolons, rhizomes and carbohydrates, rather than encouraging shoot growth.
Fall-applied N fertilization is intended to be taken up by the plant in the fall and is not a dormant fertilization (as when applied N is taken up by the plant the following spring). These researchers advise against dormant N fertilization, which has significant potential for environmental loss. (Most of the work cited in this review article was performed in Midwestern or New England states, so “fall-applied N” refers to applications made in October or November.)
Suitable nitrogen rates for fall N fertilization ranged from 1 to 2 pounds N/1,000 square feet (49 to 98 kilograms/hectare), which was 25% to 50% of the total N applied in the year. Studies that had higher N rates (> 5 pounds/1,000 square feet [245 kilograms/hectare]) as a part of the experiment found that higher rates were unnecessary and sometimes had negative consequences, such as increased susceptibility to winter injury or increased environmental damage.
Winter damage caused by fall N was a function of N rate, however. At lower rates of fall N (1 to 3 pounds/1,000 square feet [49 to 147 kilograms/hectare]), winter damage was never observed, and fall color retention and spring green-up were enhanced. In most studies, the improved color and quality was observed when soluble (ammonium nitrate, ammonium sulfate, urea) N sources were used.
Nitrogen use efficiency (percentage of applied N recovered by the plant) was typically 30% to 60%, with an average of around 50%. Much of the applied N was taken up in the thatch and verdure (shoots not removed by mowing). Lower N uptake values indicate that the turfgrass may not be good at using higher rates of fall-applied N, which could put this N at risk for environmental loss through leaching — a concern for fall-applied N.
This review of previous research found that application of fall N at rates of 0.5 to 2 pounds/1,000 square feet (25 to 98 kilograms/hectare) (lower rates for putting greens) maintained cool-season turfgrasses that retained fall color, and improved color and quality the following spring. Within those N rates, there was never evidence of winter damage. Also, no research supported the idea that late-season N either increased or decreased reserve carbohydrates. Other than these improvements in color and quality, few consistent findings support the idea of late-fall N for root growth, carbohydrate reserves or photosynthesis. Finally, the review noted that a great deal of the work was performed in Midwestern and temperate coastal climes, and the work may have limited applicability to more northern regions.
Source: Bauer, S., D. Lloyd, B.P. Horgan and D.J. Soldat. 2012. Agronomic and physiological responses of cool-season turfgrass to fall-applied nitrogen. Crop Science 52:1-10.
Editor’s note: Read all of Beth Guertal’s recent Verdure columns.
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and the 2019 president of the Crop Science Society of America. She is a 21-year member of GCSAA.